the error you are receiving it is a bug caused by some Jersey libraries compiled only for Java 7.
This has been fixed in the next upcoming 6.0.2 version. You can test it already in the available beta build from here: https://sourceforge.net/projects/jasperstudio/files/beta-release/
Plus unfortunately in Mac OS X, Eclipse/JSS was not receiving correctly the -vm flag that specifies to use the bundled JRE.
You can proceed in one of the following ways:
1) Download and test the beta build, a stable version will be available soon
2) Fix the -vm flag argument in Jaspersoft Studio.ini file to something like this: ../../../features/jre.macosx.cocoa.x86_64.feature_1.7.0.u67/jre/Contents/Home/lib/jli/libjli.dylib
You need to right click on the Jaspersoft Studio.app, "Show package contents" and navigate to Content/MacOS.
In this way JSS will be forced to use the internal bundled JRE 1.7.0.u67 preventing the error from appearing.
3) I expect that also installing a JDK 1.7 will solve the issue. Not tested yet. I suppose that in your system you are currently relying on the Mac OS X Java (that is a 1.6). You should be able to verify the available Java version giving "java -version" in your terminal.